First of all, you need to read a lot then you increase your vocabulary. Read as much as possible, newspapers, magazines, websites, books, and so on.
Definitely, if you read every day you will have conditions to improve your writing. Send emails to your friends, family, spouse, whoever. It is very important to practice day by day. At the moment you learn a new word while reading I suggest you make a sentence telling about your own experience. You need to keep in touch with the language whenever it is possible so you will be able to improve not only your writing but your listening and reading too.
If possible talk to foreigners in order to learn more words and try to talk through Skype, etc. There are many ways so you need to start anyway. Write at least one sentence a day and ask someone to correct for you. If you want, feel free to contact me: Ricardo Esteves – firstname.lastname@example.org